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2007

Piling and Archaeology An English Heritage Guidance Note

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Contents

Introduction 2 Structure 2 What this guidance note does not cover 3

Piling types 3 Displacement piles 3 Large displacement piles 3 Small displacement piles 5 Replacement (bored) piles 5 Supported replacement piles 6 Unsupported replacement piles 6 Pile-retaining walls 7 Vibro techniques 7

Piling impacts upon archaeological remains 8 Large displacement pile impacts 8 Small displacement pile impacts 10 Supported replacement piles impacts 11 Unsupported replacement piles impacts 12 Vibro techniques 12 Summary of piling impacts 13 Other issues to consider 13

Mitigating the impacts of piling – a continuing process 15 Methods to reduce or avoid disturbance to archaeological remains 15 Pile re-use 16 Understanding the zone disturbance 17 Pre-augering and probing for obstructions 17 Piling and waterlogged deposits 17 Piling and burial grounds 18 Reporting 18 Summary 18

Case studies 18 Pile pre-augering: JunXion Lincoln 18 Steel screw piles: Salisbury 19 Pile re-use: Ramada Encore Hotel, Micklegate,York 19 Pile avoidance and redesign: 43 The Highway, Shadwell, London 20

Research strategy 20 Best practice: summary of guidance 20

Early involvement 21 Pile impact 21 Pile choice 21 Pile installation 21 Records for the future 21

Contact details 22 Glossary 22 References 23 Authorship and acknowledgements 24

Introduction

In 1990 the Government published Planning Policy Guidance 16. Planning and Archaeology (PPG16) (Department of Environment 1990). This established a process for dealing with archaeological remains affected by development. A key element of PPG16 is the ‘presumption in favour of the physical preservation of significant archaeological remains’.To achieve this, building foundations are often constructed above important archaeological deposits or, in the case of piled foundations, through them. In some

cases the pile location is archaeologically excavated, particularly if the pile or pile cap is larger than 1.2m in diameter. Increasingly, archaeologists have become concerned about the impact of piled foundations on archaeological deposits. It has been suggested that piling through archaeological deposits may not have been a very effective method of in situ preservation, and caused more damage than might have at first been assumed (Biddle 1994; Nixon 1998; Davis et al 2004).

Specific issues include:

• the possibility that driven piles could damage archaeological deposits • piling carried out without effective evaluation of the site could lead to piles being inappropriately located in relation to archaeological features (so causing additional loss of information and cost to the developer) • drilling fluids and concrete (prior to setting) from bored or augered piles might leach out adjacent to the pile bore • so much of the site might be damaged that future re-examination would not be worthwhile • piling might change the site hydrology, draining waterlogged deposits

Despite this, foundation solutions that preserve the majority of an archaeological site in situ are an essential tool in ensuring that development can take place where archaeological remains are present. It is acknowledged that foundations might damage buried deposits, but this is part of a balanced trade-off between allowing development to take place and the protection of the majority of the archaeological deposits of a site. This ‘balanced trade-off ’ is a fundamental assumption underpinning this guidance note.The information in this document is provided to ensure that where damage does occur, the impact on deposits and artefacts is minimised.

Structure This guidance note has been prepared to assist planning and archaeological officers, and developers and their consultants to make well informed decisions about piling schemes and the potential impact upon archaeological remains. It provides information on piling types and their impacts, with mitigation recommendations, a research strategy and case studies.

Piles, and the main piling types, are covered in the first section, describing the piling techniques used to construct foundations, and the engineering choices and constraints, to enable consideration of these techniques within proposed in situ preservation schemes. The potential impacts of each pile type on archaeological deposits are then considered. This section is followed by discussion of how to mitigate the impact of piling, giving a range of options.These focus on the types of decisions that planning and archaeological officers, developers, and their archaeological consultants need to consider throughout the design and construction process. Case studies are provided to demonstrate some of the mitigation suggestions. Future research priorities are also identified; while the guidance note and case studies address many of the potential effects of piling, and offer generic solutions, it is recognised that comprehensive information is lacking on several key archaeological issues.

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What this guidance does not cover This guidance note is only concerned with piling impacts; it does not cover other impacts associated with pile construction, such as piling mats or the excavation of pile caps, or other aspects of construction.These are covered in study of the Mitigation of Construction Impacts on Archaeological Remains (Davis et al 2004), which provides information on pre-construction, construction and post-construction impacts on archaeological remains. It is also assumed that the use of piled foundations as a means of mitigation is a choice that is taken in the context of other construction, planning and archaeological mitigation scenarios, including the refusal of planning permission for the construction of a particular development, the use of shallow foundations, the redesign of any structures to avoid below-ground disturbance or preservation of the site ‘by record’, through archaeological excavation.

Piling types

Piling is a method of transferring load from a structure into the ground.The engineering objective of a pile is to support a structure by using the strength of the ground some distance below the surface that can resist the imposed force.This can be by direct bearing onto a firm stratum present at depth below the site or by using the frictional resistance of the soil against the pile shaft to develop the load-bearing capacity. In some cases a combination of these is used where the pile is founded on a firm horizon and the sides develop surface friction (Figs 1 and 2).

Engineering factors influencing the choice of pile type include:

Fig 1 (top left) End bearing pile, where the pile is founded in the hard incompressible layer rather than the soil above.

Fig 2 (top right) Friction bearing pile, where the sediment becomes increasingly stiff with depth.

• the proposed structure and location (for example high-rise urban flats or low-rise greenfield warehousing) • ground conditions (ie cohesive or non-cohesive soil) and location of the water table • durability (for example, concrete can suffer chemical attack and steel piles may corrode) • cost (including speed of installation and certainty of the chosen method being effective)

Pile types in this guidance note are grouped and described following Tomlinson (1994) (Fig 3).

Fig 3 (middle) Pile types (after Tomlinson 1994, ch 8).

Displacement pilesDisplacement piles push the sediment aside as they are installed, compressing the ground and increasing the bearing capacity of the foundation. Displacement piles are environmentally positive in the sense that there is no need to remove spoil, no landfill requirements, and reduced vehicle movements. This is particularly important on contaminated sites where the arisings (spoil) would require remediation. There are several forms of displacement pile (Fig 4).

Fig 4 (bottom) Displacement pile types (after Tomlinson 1994, ch 8).

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Large displacement piles Large displacement piles can be constructed from concrete, metal or, rarely, wood, and are installed by hammering, jacking or vibrating the piles (or tubes) into the ground (Fig 5). A drop-hammer simply drops a large weight onto the top of the pile, however, they produce significant vibration. Hydraulic hammers use a controllable powered ram and are quieter and cause less vibration than the drop-hammer. If

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sediments are soft, preformed piles are jacked rather than hammered in, which has the advantage of being quiet and effectively vibration-free.

Fig 5 (above top) Displacement pile installation: Piles arrive at site, pile located, pile section driven, additional section attached, pile driven (image courtesy of Cementation Foundations Skanska).

Driven preformed piles Solid piles are usually constructed from precast concrete (and occasionally wood) and come as specific lengths or sections joined together to form a longer pile (up to c 40m). Low headroom rigs can therefore be used in areas of restricted access.The normal range of preformed concrete pile sizes in the UK is 150 to 300mm

2.The advantage of using

preformed concrete piles is that there is no need to wait for concrete to set, nor for liquid concrete to be transported to, or prepared on, site.The pile sections can be coated before insertion to prevent reaction with the surrounding soil and improve concrete durability.

Hollow piles are tubes generally constructed of steel or occasionally precast concrete.

Fig 6 (above right) Preformed concrete displacement pile being installed, photograph courtesy of Mike Brown..

(Fig 6)

The concrete may be prestressed to enhance durability. Hollow piles are often used when large diameters (>500mm) are needed and are hollow for ease of handling, or for economy. For hollow steel piles, concrete is poured into the hollow section to complete the pile (as for driven cast in situ), except that in this case the tubes are not withdrawn.

Driven cast in situ This method is used less often than driven precast piles (Figs 7 and 8). A tube (steel or precast concrete) with a sacrificial shoe or detachable point is driven into the ground, displacing and compacting the soil around the tube. Reinforcement is lowered into the tube and concrete poured into it. As the concrete is added, the tube is withdrawn and the concrete may be compacted.This method is normally used to create piles from about 250–500mm diameter with depths of up to 25m.

This method is particularly useful in contaminated soils, because no arisings are produced; however, removal of the tube can cause distortion of the surrounding sediment and may allow movement of liquid concrete into voids.

Fig 7 (above) Illustration of driven cast in situ pile installation: tube located and driven into ground, filled with concrete, tube withdrawn leaving completed pile (image courtesy of Cementation Foundations Skanska).

Fig 8 (right) Cast in situ pile: having been driven, the concrete is being poured down the metal tub (photograph from Cementation Foundations Skanska).

Auger displacement piles This method uses a spiral auger that displaces the spoil laterally into the ground around the hole. Concrete is poured down the auger shaft as the auger is withdrawn.The displacement consolidates the ground surrounding the pile, resulting in enhanced soil properties and therefore shorter pile lengths.

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Pile sizes will depend on the individual pile company’s specific auger design, but diameters of 300mm to 600mm are likely.This type of pile is relatively ‘green’, its installation producing very little spoil, vibration and noise.

Small displacement piles Preformed steel Small metal piles (H-section, sheet, tube or box) are hammered, vibrated or jacked into the ground. Sheet piles (Fig 9) are often constructed as interlocking piles, used to create cofferdams or retaining walls, and less often to support load from a structure above.Where they are used for retaining walls, sheet piles may also need tie-backs, which will have a further impact on adjacent deposits.

Smaller metal piles include rolled steel sections, screw piles and H-section piles. Rolled steel section piles are easily handled and can be driven hard, and in very long lengths; and while the pile length can be readily varied, lengths of up to 36m can be achieved.

Screw piles can reach 24m in depth, carry heavy loads and can be successfully anchored in steeply sloping rock surfaces.The piles are liable to corrosion, which can be treated using cathodic protection, or a pile coating. Steel screw piles, consisting of a number of curved spirals of steel connected to a central shaft, have been recently used on an archaeological site and this is cited as a case study.They are easily removable, and are likely to cause minimal sub-surface disturbance.

Fig 9 (top) Sheet pile retaining wall along right edge of site, courtesy Pre-Construct Archaeology Ltd.

Fig 10 (middle left) Rolled steel tube being installed at Skirbeck Road, Boston.

Engineering advantages and disadvantages of displacement piles The advantages of displacement piles lie in the controlled and clean nature of the system (Fig 11). No spoil is produced and piles are generally preformed with no need to transport or make fresh concrete on site, except when casting in situ. Piles can be quickly constructed in variable and long lengths, (also in low-headroom areas) and are unaffected by the presence of groundwater. Additionally, off-site production in controlled conditions means the preformed sections are constructed to a higher and more uniform specification than is possible with on-site piles cast in situ. Screw piles are valuable in marine works because they can resist both tensile and compressive forces, and were extensively used by the Victorians in the construction of seaside piers. In general, small driven piles and metal screw piles are particularly useful if ground displacements and disturbance must be curtailed.

Disadvantages with displacement piles include breakage below ground, and the difficulties of checking pile quality. Soil displacement can cause heave, and lift or damage adjacent piles or damage adjacent buildings.The noise and vibration associated with pile installation can be considerable, and

makes this method unsuitable in built-up areas and adjacent to fragile historic structures.

Fig 11 (middle right) Displacement pile being installed in Boston.

Replacement (bored) piles Replacement piles are installed by boring a hole, removing the arisings and filling the hole with concrete (and often reinforcement) (Fig 12). The bore tends to consist of a screw-type auger on a piling rig, which augers directly into the ground and removes arisings in a series of passes, using a ‘flighted’ or bucket auger. Light cable percussion bores (shell augers) are also used to construct a hole, using a hollow cylinder which is hammered into the soil, and then the enclosed soil removed before another length of cylinder is added and pushed further into the ground to continue deepening the hole. Piles are usually cast in situ or occasionally constructed using pre-cast concrete ring sections, which are then filled with concrete. Piles can be constructed with diameters of up to 3m, and can be bored to depths of up to 70m, with under-reamed bases up to three times the shaft size. Small diameter bored piles are usually less than 600mm diameter and can reach 30m in most ground conditions. Bored mini-piles are of the order of 200–300mm in diameter and reach up to 15m deep.

In some instances a casing is inserted, usually temporarily, to prevent the collapse of the hole, and the auger drills inside this. In the case of continuous flight-augered (CFA) piles, the arisings are removed at the end of the operation when the auger is removed, making support unnecessary.With any of the

Fig 12 (bottom) Illustration of rotary bored pile construction.The auger is located and begins to remove soil, a temporary casing is installed to prevent the upper deposits collapsing, further material is removed, the concrete and reinforcement are added, the casing is removed and the pile is complete (image courtesy of Cementation Foundations Skanska).

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replacement piling methods, there is typically little or no sediment displacement adjacent to the shaft of the pile. Increased pile capacities can be achieved through the formation of enlarged pile bases (under-reams) (Fig 13).

Fig 13 (left top) Replacement piles (after Tomlinson 1994, ch 8).

Supported replacement piles In unstable soils a casing or a drilling mud/slurry, such as bentonite, may be used to support the borehole walls.The choice between using metal casing or slurry is an engineering decision; generally casings are used to line a relatively shallow depth of unstable ground to reach a self-supporting stratum below, while a drilling mud is used to support unstable ground at lower levels.

Temporarily supported A drilling mud such as bentonite would be used when piling through a deep, unstable stratum and subsequently pumped out.The use of bentonite has specific implications, including adverse environmental effects and the large space required for bentonite plants on site. Additionally it may be classified as controlled waste, in which case disposal requires special precautions and additional expense. Synthetic polymers may be used instead of bentonite, although their use is currently less common. Pile casings are generally metal tubes inserted into the ground by driving, vibration, oscillation or rotation. Noise and ground vibration can be high where a casing is installed.These levels, however, will generally be much less than for driven pile installation, although tripod-bored piles can also produce significant noise and vibration. Sometimes casings are installed by ‘mudding in’, the contact between the casing and soil being lubricated using bentonite.This can significantly reduce the noise and vibration effects. Most casings are removed after the pile

has been formed, although some are left in place permanently, even though this adds significantly to the cost.

Unsupported replacement piles Continuous flight auger (CFA) The CFA technique is one of the most commonly used piling forms and can be used in most soils.The hole is augered out and high slump concrete is pumped into the hole through the auger shaft to the base (Figs 14 and 15).

As the concrete is inserted, the auger is withdrawn, taking the arisings with it. A reinforcing cage can then be pushed into the liquid concrete. Practically no vibration or noise is created using this piling technique. Pile diameters are usually 0.3–1.2m and they can reach depths of 25m. Casing is rarely needed as the sides of the bore do not need supporting as the arisings are not removed until the concrete is pumped in.

Fig 14 (above bottom) Continuous Flight Auger (CFA) piling (image from Cementation Foundations Skanska).

Fig 15 (left bottom) Illustration of CFA pile construction: The auger is located and rotated into the ground to the desired depth, as it is withdrawn the concrete is added, and finally reinforcement is added and the pile is complete (image courtesy of Cementation Foundations Skanska).

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Fig 16 (above top) Photograph from above a CFA pile during the construction of a pile. Soil can be seen in the lower flights, and around the auger where it has been cleaned off.The reinforcement cage stands adjacent (left) (image from Cementation Foundations Skanska).

Engineering advantages and disadvantages of replacement piling The benefits of using replacement piles include the variability of length and diameter, the low risk of ground heave resulting from pile installation, and the low noise and vibration.

Disadvantages include the need to bring liquid concrete to site, or create concrete/bentonite plants on site. A further disadvantage is that CFA piles cannot be inspected once cast. For bored piles where a drilling mud has not been used, the open pile bore can be inspected before placing of concrete, so the length, depth, shaft, and base quality and verticality can be easily verified. Support fluid or casings are usually required to construct bored piles in

unstable sediments and the transport, use, storage and disposal of these materials and fluids all need to be taken into account.

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Pile-retaining walls Bored pile-retaining walls are created by drilling a line of holes and forming piles either as contiguous or interlocking (secant) sections (Fig 17). Secant walls are drilled in two phases – primary piles, then secondary piles that partly cut the primary piles.They are often used to retain the surrounding ground as well as for their high stiffness and water-retaining properties. Contiguous pile walls will not retain water, but are cheaper than secant walls.These types of pile are generally between 0.45m and 3.0m in diameter and can reach lengths of 60m. In virtually all cases guide trenches are constructed before secant (but not necessarily contiguous) walls are created in order to remove obstructions and create the line.This will therefore remove soil, which might then need to be taken from site. Pile-retaining walls are not always used to support a building, but to contain lateral stress, for example within basements.

Fig 17 (top) Secant pile wall in background, from Gresham Street, London (© MoLAS).

Vibro techniques Vibro techniques are a form of ground improvement rather than piling. However, from the archaeologist’s point of view, vibro methods present similar problems and so are briefly considered here.They use densification and/or the insertion of stone or concrete columns to provide greater below-ground stability prior to

construction. Key techniques are vibro compaction and the creation of columns using displacement and replacement methods, such as vibro replacement (Mitchell and Jardine 2002). Dynamic compaction involves dropping a large weight onto the ground and should not be confused with vibro compaction.

Vibro compaction and vibro replacement – stone columns Vibro replacement methods are used in mixed cohesive, granular or purely cohesive soils, particularly weak soils and fill. A vibrating poker is used to create a hole into which stone

aggregate is inserted and vibrated to bond with the surrounding soil.Vibro compaction is rarely used in the UK; it requires purely granular soils with low silt content.Vibro compaction uses a vibrating poker (often 300–400mm diameter), inserted into granular soils to agitate and compact them; water is often used with this system to remove very fine particles, and assist in penetration (Fig 18).

Fig 18 (bottom) Bottom feed vibro replacement (images courtesy of Keller Ground Engineering).

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Vibro Concrete Columns [VCC] Concrete columns [VCC] can also be constructed using vibro techniques. A vibrating poker creates a void, usually through weak soils and is founded on a solid layer. Once the void is created by horizontally and vertically displacing the soil, a very low slump concrete is pumped into the hole through the poker (Figs 19 and 20).

Figs 19 and 20 (far left) Bottom-feed vibro replacement (photographs from Keller Ground Engineering).

Engineering advantages and disadvantages of vibro techniques As columns of stone or concrete are inserted to create a support grid within the soil, this increases ground-bearing capacity without generating spoil and so is considered environmentally ‘green’. Additionally, although ‘stone’ columns are often aggregate, recycled ballast is now regularly used, furthering sustainable development. A high-density grid of vibro columns is particularly useful where increased load-bearing is required. When stone columns are used as foundations (rather than for ground stabilisation), more columns are usually required than piles.

Piling impacts upon archaeologicalremains In this section, the impact of each of the pile types is explored, detailing physical and hydrological impacts upon archaeological remains. All techniques result in damage to or loss of artefacts, and in sediment deformation equal to at least the total volume of the pile or vibro-replaced column.This is the minimum impact that will result from any piling operation. In many cases, further disturbance may occur, and the extent of

that disturbance must be understood in order that the impact and implications of foundations and piling schemes can be assessed.Additionally, hydrological impacts on the deposits may affect the deposit/groundwater chemistry and microbiology. This is not only relevant on waterlogged sites, as changes in deposit hydrology and chemistry can affect inorganic as well as organic remains.

Large displacement pile impacts Driven preformed piles: physical impacts Physical impacts of driven preformed piles on archaeological remains have been recognised for a number of years (Biddle 1994; Dalwood et al 1994). During pile installation, sediment is physically displaced vertically and horizontally, which can cause distortion and damage to archaeological deposits, structures and artefacts. Such displacement is demonstrated in the image from Farrier Street,Worcester (Dalwood et al 1994), which shows down-dragging of deposits resulting from pile installation (Fig 21). Dalwood et al suggest, (on the basis of calculations made from excavations adjacent to piles in Worcester) that the area of the site affected by piling operations was up to six times larger than originally predicted. Although numerous anecdotes of pile damage exist, few have been published. A survey of 46

Sites and Monuments Records for reports of piling impacts produced only three examples (from 17 replies) where piling impacts had been recorded (Davies 2004). At the Marefair, Northampton, significant distortion was recorded adjacent to one of the piles (480mm in diameter), with disturbance up to 250mm either side.The total area of damage had a radius of approximately 1.0m and vertical displacement of over 1.0m, (Northamptonshire Archaeology undated).

Unfortunately, while the characteristic inverted-cone resulting from down-dragging had been recorded, little is known about which pile installation technique was used in the past. It is therefore impossible to be sure,without going back to the original piling records (where they survive), whether such examples result from driving preformed piles.The pile excavated in Northampton was circular, and may not have been a preformed displacement pile.The same questions apply to Roman deposits at Vine Street in Leicester, which demonstrated similar sediment distortion associated with a circular pile (Fig 22).The rough external surface of the pile suggests that it was a bored pile rather than a solid preformed pile, although it may have been driven, then pressure grouted.

Fig 21 (above) Image of sediment deformation adjacent to piles (from Dalwood et al 1994; © Worcestershire Archaeological Society and Worcestershire Historic Environment and Archaeology Service).

Fig 22 (left) Layered deposits deformed by piling at Vine Street, Leicester (image courtesy of the University of Leicester Archaeological Services (ULAS)).

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Another example comes from Number 1 Poultry, London, where piles installed in the 1970s were recorded during later excavation. Figure 23 shows a pile penetrating a Roman mosaic, which is undamaged outside the pile footprint (Rowsome 2000).The exact type of installation method is unknown, but again the surface finish of the piles is rough.

Waterlogged archaeological deposits are at great risk from driven piling, although much would seem to depend upon the orientation, and state of preservation of surviving timberwork, in particular. Significant damage is reported from Finnegården 3A in Bergen,

Norway (Biddle 1994) (Fig 24), and the Thames Exchange site, where waterfront timber revetments show damage up to three times the diameter of the pile (Nixon 1998, 42 and fig 2). More limited deformation of deposits was reported by Stockwell (1984) from soft organic-rich deposits from Coppergate, where piles cleanly cut through waterlogged timber without significant levels of down dragging.

Unfortunately, there has been no clear requirement for archaeologists to collect piling data from redevelopment sites in any rigorous way. In many instances, evaluations have consciously avoided areas adjacent to piles because they are likely to be disturbed (Davies 2004).This results in vital opportunities to understand the past impacts of construction being missed.This should be a basic requirement on any excavation where previous foundations are encountered.

Fig 23 (top) Pile and mosaic from No 1 Poultry (© MoLAS).

Fig 24 (above) Impact of a driven pile on deposits at Finnegården 3A in Bergen, Norway, where dragged-down sediment layers and displaced wood are visible next to the pile (© Norwegian Directorate of Cultural Heritage, Excavation unit Office West, 1982; photograph by Karsten Kristiansen).

Fig 25 (left) Typical result from model testing in layered ground, showing vertical displacement of the clay layer (middle) by the installation of a pile (Hird et al 2006 Figure 4.9).

Fig 26 (left below) Image of homogeneous sediment deformation, the composition of the sediment is 75% sand, with 25% kaolin clay. Marker layers are included to allow displacement to be recorded (image courtesy of Keith Emmett).

Engineering and field scale research Down-dragging of sediment is also relevant to engineers, and several model-scale experiments have been carried out to characterise the extent of deformation. Most of these studies show a drop-off in visible sediment movement within about 1.5 pile diameters of the centre line of the pile (Hird et al 2006, particularly fig 1).This research was carried out predominantly on homogeneous clay soils, which may not effectively replicate all archaeological deposits. Model-scale (1:10) research on driven and CFA piles in layered soil has provided information on the mechanisms of sediment displacement and the extent of the impacts. Figure 25 shows the typical extent of sediment distortion recorded in a model-scale experiment. Samples were tested in both consolidated and unconsolidated models, mostly with a clay layer sandwiched between two sand layers, with variable layer thickness and density. Some homogeneous samples with varying mixes of clay and sand, containing marker layers for identification of sediment displacement were also used (Fig 26). Although a number of the tests in this work were on unconsolidated sediments (including both shown here), the results and data are physically and numerically similar to the tests on consolidated deposits that were also produced, and to the results from previous work (Hird and Moseley 2000). In almost all instances the maximum extent of deformation lies within 1.5 pile widths of the centre line of the pile, although ‘most of the vertical displacement (or down-dragging of soil) is concentrated within a distance of 1 pile width from the pile centreline’ (Hird et al 2006).

Field-scale evaluations have been carried out to test the extent of pile damage to archaeological deposits. At the JunXion, Lincoln, two 0.25m­long square preformed replacement piles were installed and evaluation trenches excavated alongside to investigate the degree of sediment deformation.The excavation demonstrated that

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sediment deformation had occurred adjacent to the driven pile, but this was only visible within 0.1m of the pile edge (less than one pile width from the centreline).The down-dragging effect had nevertheless extended 1m down, clearly seen with different coloured material (Figs 27 and 28). Other visible effects included cracking, remoulding of deposits and the creation of voids (Davies 2003). It is unfortunate that the deposit was homogeneous fill because deformation features were not particularly clear.

Excavations were also carried out beside four piles at Skirbeck Road, Boston, Lincolnshire. These included three preformed concrete piles (one of which was pre-augered, and another was fitted with a pointed shoe), and a hollow steel pile (see Fig 10). In all cases, sediment deformation was difficult to make out owing to the complicated nature of the stratigraphy. All of the visible impacts were within 1.5 pile widths of the pile centreline, and in several cases, significantly less (Rayner 2005).

Fig 27 (left) Piling mat carried down adjacent to the pile, the JunXion, Lincoln (photograph courtesy of Glyn Davies, ARCUS).

Fig 28 (middle) Section drawing of sediment deformation from the JunXion, Lincoln (Davies 2003).

Driven preformed piles: hydrological impacts As driven preformed piles are constructed off-site, the potential impact on deposit hydrology and geochemistry is likely to be less than where the pile is cast in situ. The compression of deposits adjacent to the pile should lead to a reduction in permeability in this area, thereby reducing hydraulic conductivity of sediments at the soil/pile interface. However, where piling occurs through perched water-tables, there is a

potential for dragged down and deformed deposits to create a pathway for downward migration of water, and cause drainage of previously waterlogged deposits. Model-scale tests suggest that there is no significant increase in permeability for driven piling in layered sand and clay samples, providing the impermeable (clay) layers are relatively soft and sufficiently thick, that is, more than two pile diameters thick. Changes do occur, however, where there is a thin clay layer relative to the pile diameter/width, which is exacerbated in the case of H-section piles (Hird et al 2006).These model-scale studies also demonstrate that small amounts of contaminants could be carried down at the pile toe but, in the absence of the creation of any long-term preferential pathways for further contamination, the impact that limited amounts of contaminant will have on archaeological deposits and artefacts is not likely to be excessive.

Recent excavations in Spurriergate,York have revealed extensive waterlogged deposits dating from the Roman and Anglo-Scandinavian periods. Much of the site had previously been piled using square-section preformed concrete piles. In one area of Roman dumping, there was a clear zone of impact around each pile, and the sediments appeared much drier than the surrounding deposits. In another area, however, identical piles had been driven through a possible Anglo-Scandinavian timber building and organic-rich deposits showing no zone of impact around each pile. Equally, where concrete displacement piles were driven through Bronze Age timbers at Bramcote Green in London, the timbers were almost entirely destroyed; where there were no piles, the timbers were intact (T Nixon pers comm).

Driven cast in situ piles: impacts The physical impact of driven cast in situ piles is similar to driven piles, that is, vertical and

horizontal displacement of deposits. It is possible that further modification of deposits occurs when the casing is removed. Currently, there has been no evaluation of this, so caution should be applied in assessing the likely damage using this technique. Aside from the physical impact associated with the removal of the tubing, if the pile grout is still liquid, it could escape into any voids.These might be present in poorly consolidated deposits, or perhaps in fissures within the sediment. In waterlogged deposits, there is a risk that chemical interaction will occur between the pile grout and archaeological remains.This is discussed in more detail within the section on replacement (bored) piles below.

Screw displacement piles: impacts No evidence exists about the physical impacts on archaeological remains from screw displacement augers.This technique may be more damaging than replacement piling, because the displacement auger forces the sediment aside, leading to sediment deformation in the vicinity of the pile.The sediment adjacent to the pile will have been compacted, decreasing permeability at the soil/pile interface, relative to a replacement pile. Therefore, potential impacts, discussed in more detail for replacement piles, such as grout migration are less likely to occur. However, this is an area where further research is needed to characterise the nature of below-ground soil movement.Without a firm understanding of the likely zone of deformation, screw displacement piles should only be used within a pile mitigation programme following a full impact and risk assessment.

Small displacement pile impacts Preformed steel

Fig 29 (right) H-section pile showing re-entrant angle (© Trace Parts S.A. www.traceparts.com).

H-section piles have a smaller cross-sectional area, and therefore, in theory, should lead to less sediment displacement than square preformed driven piles (Figs 29 and 30). Although no field investigations have confirmed this, model-scale analysis has shown that there is a reduction in the amount of vertical deformation of deposits (Hird et al 2006). However, in tests with a clay layer between two sand layers, sand can be seen to plug within the re-entrant angles of the pile, and is carried down into, and possibly through the clay layer. This allows movement of liquid along the pile. This partly confirms previous research on

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H-section piles (Hayman et al 1993; Boutwell et al 2000).The potential corrosion of these piles and any effects this might have on archaeological deposits are the same as for driven sheet piles (see below).

Sheet piles also have a limited cross-sectional area and the amount of material displaced during installation will be significantly lower than other pile types. Sediment deformation is therefore most likely to occur where

obstructions are encountered, and archaeological material is dragged down, or the original orientation of materials is altered. In many cases though, sheet piling will cut through archaeological materials.The installation techniques used for sheet piling, including impact and vibro driving can induce ground vibrations that might damage fragile archaeological materials or adjacent buildings.

A specific concern with steel sheet piles is corrosion. A number of studies have been carried out on steel piles, which show very limited levels of corrosion occurring within the ground, within anoxic saturated soils (see for example reviews in Morley 1978 and in Tomlinson 1994, ch 10, particularly 10.4.1). Fewer studies have looked in detail at the potential corrosion associated with soils above the groundwater table.Where data exist, it is clear that corrosion is enhanced in disturbed soils with a constant oxygen supply, and also on contaminated sites. It is possible that corrosion of metal piles may damage archaeological materials when corrosion products are transported into other parts of the deposit in solution through surface water/groundwater percolation, although the risk is fairly low.The use of plastic sheeting or pre-treatment of metal piles would avoid issues associated with pile corrosion.

Where sheet piles are used to create an impermeable barrier (such as a cofferdam), then de-watering may occur. However, recent excavations in Bergen, Norway, have shown that substantial water flow occurred through a small hole in the pile (Matthiesen 2005). An investigation of the state of preservation of material on either side of the sheet piling indicated that there was no significant difference.

Steel screw piles will have minimal physical impact on archaeological deposits (where obstructions are avoided) and have the added benefit that they can be unscrewed when they are no longer required, a process that should also involve little damage to deposits (Fig 31). The main impact will be the displacement of material during insertion. Additionally, if obstructions became caught between the pile blades, then this could lead to further disturbance. Since some compaction of the ground adjacent to the pile will occur, the pile is unlikely to act as a major conduit for migration of water or contamination within archaeological deposits.There is potential for corrosion of the pile above the groundwater table, which may have an impact at the time of pile removal if corrosion products have become integrated with the surrounding soil or archaeological material, which may lead to greater disturbance as the pile is removed.

Fig 30 (top) H-section pile test with sand plugged within the flanges of the pile (Hird et al 2006 Figure 4.2a).

Fig 31 (middle) Small screw piles in advance of installation in Salisbury (see case study) (photograph courtesy of Tim Sheward, Roger Bullivant).

Fig 32 (bottom) Loss of material during bored piling operations at the level of the watertable, at Number 1 Poultry, London (© MoLAS).

Supported replacement pile impacts Temporarily supported bore: physical impacts An accepted impact associated with conventional bored piles is the loss of material from within the cross-section of the bore. In

principle, there should be no disturbance of material adjacent to the hole, but this is negated if the auger encounters large cohesive items that are forced outward or dragged down through significant deposits outside the intended bore.

Few published examples exist where archaeological evaluations recorded details of replacement piles. In excavations next to new piles installed at Number 1 Poultry, about 7% of the bored piles caused significant damage at the level of the watertable, with an area twice the diameter of the pile being affected (Nixon 1998, 41).This may have occurred during the installation of the pile casing as the damage was only seen next to (some of) the supported replacement piles, but not next to unsupported CFA piles (T Nixon pers comm).The impact is shown in Figure 32, with loss of an area of beaten earth floor adjacent to the pile (Rowsome 2000).

During the installation of temporary or permanent casing vibration may occur, and the impact of this, in addition to that of the installation and removal of the casing, has not been fully evaluated.There is a potential risk, highlighted by Nixon (1998), that the installation and removal of the casing may damage an area greater than the diameter of the casing itself. As temporary casings are usually installed to support poorly consolidated deposits, this should reduce any collapse of the bore walls or migration of pile grout into sediment voids.These concerns should be discussed by archaeologists and piling engineers on a site-by-site basis.

Other physical impacts may occur where stones, timber and other materials are not cleanly severed by the bore or casing and are pushed aside or dragged down (Nixon 1998, 41). It is possible to get borers capable of cutting through brick and soft stone and it is essential that the likelihood of encountering such sub-surface ground obstacles is clearly addressed in the site evaluation and mitigation strategy, because unforeseen obstructions may hold up the construction programme, and necessitate excavation to remove them.This excavation can be exceptionally damaging to archaeological deposits, and can mean that much a greater area of the site is affected than just the pile locations.

Where bentonite (or synthetic polymer) is used to support unstable sediments consideration should be given to the impact of this on archaeological deposits.The complexity of the operation means that a compound often needs to be constructed on site for the slurry processing plant. Bentonite is inert so it should pose no chemical risks to archaeological deposits, however, the physical impact on archaeological remains has not been fully considered, so it is recommended that an impact assessment is carried out before use. Where the site is likely to contain voids or the

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archaeological deposits are poorly consolidated, there is an enhanced risk of the slurry entering these areas. In these cases, a temporary casing should be used for the depth of the archaeologically sensitive deposits.

Temporarily supported bore: hydrological impacts There is a potential risk that the introduction of an alkaline mixture (concrete) will damage archaeological deposits, particularly waterlogged ones. Concrete curing is exothermic (Davis et al 2004), the heat potentially acting as a catalyst for further reactions (cf Edwards 1998).The potential for mixing of grout and groundwater and for transport of alkaline solution across a greater proportion of the site has yet to be fully evaluated.Where concrete cures quickly and bonds with the sediment of the bore wall, permeability and the potential for transport of alkali materials from the concrete in the groundwater should be reduced.This is a topic where more research is needed, particularly in places where the hydraulic conductivity of the deposits is high, and the movement of groundwater is therefore fast.Two field-scale tests were carried out (on CFA piles) to look at the extent of chemical contamination of deposits from pile cement entering into solution in the groundwater (Hunter et al 2006; Langdale-Smith 2006) (Fig 33). At each site, a single pre-piling core sample was taken in the location of one of the piles. After pile installation, two core samples were taken at distances from the pile, in the direction of

groundwater flow. Sediment and chemical analysis was carried out on each of the samples, to compare before and after the piling. The samples were analysed for a range of metals, anions, electrical conductivity and pH.

At both sites, minor variations in the results between the pre- and post-piling samples were recorded. It is uncertain whether these changes were due to the piling or merely represent natural variability of the burial environment.This is still therefore an area where more research is recommended.

Fig 33 (left) Borehole rig with CFA piling rig in the background, during sample retrieval to investigate pile cement migration, Slipper Baths, Lincoln (photograph courtesy of Mark Allen).

Unsupported replacement pile impacts Continuous flight auger (CFA): physical impacts Provided cohesive materials are cleanly severed, CFA piling should not damage deposits outside the area of the auger. This is confirmed by model-scale research with a fully flighted auger where impacts outside the diameter of the pile were ‘relatively small compared with’ driven circular, square- and H-section piles (Hird et al 2006). Comparison can also be made with field-scale re-excavation of preformed driven piles where both direct-driven and pre-augered piles were studied to assess whether pre-augering was an appropriate mitigation measure to reduce vertical sediment displacement (Davies 2003; Rayner 2005). In that case it was shown that there was no impact outside the diameter of the auger (Fig 34), and this may therefore apply in principle to CFA augering.Where archaeological deposits contain structural material (bricks, stone, wood) then these obstructions may be dragged within the auger flights and damage adjacent deposits.

An additional benefit of CFA piles is that the soil remains within the auger flights until the concrete is injected, which significantly reduces the potential of pile wall collapse. If, however, an auger is rotated at an incorrect speed,

adjacent material can be drawn into the bore (under-flighting), or material within the borehole forced into adjacent deposits. There is still a risk that cement will migrate into any voids adjacent to the bore.The hydrological and geochemical impacts are similar to those discussed for supported replacement piles (above).

Fig 34 (right) Pile installed into pre-augered hole at Skirbeck Road, Boston.The installation has not deformed the layers, and the edge of the borehole can be seen to the left of the pile (image courtesy of APS).

Vibro Techniques Vibro replacement: physical impacts One of the principal disadvantages of vibro replacement is that material is forced into the ground, displacing sediment (and archaeology). As the process involves vibration, the soil adjacent to the column is considerably disturbed during the displacement process and this is likely to have a very significant impact on adjacent archaeological deposits. Furthermore, columns are usually installed at around 1.5m to 3.0m c/c (column centres) so there tend to be more replacement columns on a site than if it were piled, increasing the frequency of any impacts.This is a recent technique, so there have been few opportunities for archaeologists to evaluate its effects.

Vibro compaction and vibro replacement: hydrological impacts Where vibro replacement stone columns are constructed, although these are extremely dense, there is a potential that they could act as conduits for the movement of contaminants, moisture and fluids. In such conditions a concrete plug is generally installed to avoid the dispersion of contaminants.Where the hole created by the vibrating poker is filled with concrete rather than stone, the potential for grout migration will be very limited, as any voids are likely to have been consolidated by the initial vibration. Given the extent to which the physical impacts from vibration may have disturbed any adjacent archaeological deposits, consideration of hydrological impacts may be of limited consequence.

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Summary of pile impacts on archaeological deposits and artefacts Table 1 contains a summary of the information outlined above. Methods of mitigating impacts are given in Section 4, below.

Table 1 Summary of pile type impacts

Pile Type Lateral Sediment Concrete migration Creation of preferential Vibration (noise & Metal Corrosion (of piles) displacement pathway sediment movement)

Displacement piles Yes No (possibly for Driven Not usually, except in Yes, can be reduced Yes with steel sheet and (large and small) cast in situ piles) thinly layered ground H section

and with H section piles

Auger displacement piles Yes Low potential Low potential Limited No

Replacement piles Low potential Moderate potential, Low potential Limited, but more likely No reduced by casing where casing is used (except for CFA)

Vibro compaction & vibro Yes No Low potential Yes No replacement – stone

Vibro replacement – Yes Low potential Low potential Yes No concrete

Other issues to consider Vibration Vibration from piling can affect above-ground structures as well as below-ground archaeological deposits (Fig 35). Limits on the acceptable levels of vibration from piling in relation to above ground structures are given in the British Standard (BS) 5228 part 4 (1992, 23). A conservative threshold for minor or cosmetic damage should be taken as a peak particle velocity (ppv) of 10 mm/s, for frequencies between 10Hz and 50Hz.These levels relate to soundly constructed residential properties and similar structures that are in generally good repair. BS 5228-4 also notes that ‘special consideration should be given to ancient ruins and listed buildings’. An appendix gives summary case history on vibration levels measured on site for a range of piling and ground improvement techniques, for a range of deposit types. More detailed guidance on vibration from piling on above-ground historic structures is provided in a CIRIA technical note (Head and Jardine 1992). It summarises a number of other country codes, including the German DIN 4150, as well as Swiss and Swedish standards and codes.The simplest guidance is given below, after DIN 4150 (1970).This provides levels of vibration for specific types of buildings: For structural monuments, particularly those in less than prime condition, category I (and

possibly II) are relevant. Historic buildings, which are built to different specifications than modern well-stiffened buildings should be covered by categories II and III. If vibration from piling is likely to be an issue on site, a more detailed assessment should be made, considering frequency of vibration, ground conditions and the type of building and its foundations (see Head and Jardine (1992, 41–6).

Vibration can also affect archaeological materials below ground, and intense vibration through soil can damage stratigraphy and embedded artefacts (Sidell et al 2004). This can be caused by pile installation, dynamic pile testing, and ground improvement techniques such as vibro compaction. Additionally, vibro piling hammers generate high amplitude vibrations during start-up and close-down.The vibrations from the pile travel both laterally and vertically (Fig 36).

Fig 35 (top) During the installation of piles adjacent to the Scheduled Ancient Monument of Hussey Tower, Skirbeck Road, Boston, vibration monitors were installed to ensure that vibration did not exceed the agreed limits.The pile locations were pre­augered in part to reduce ground vibration.

Fig 36 (bottom) Vibration monitoring during driven piling using geophones, as part of the NERC Urgent project (see Sidell et al 2004).

Pile size and geometry Piling requirements on individual sites will relate directly to the structural needs of the building, and the strength and compressibility of the below ground deposits. Since soils behave differently it is difficult to generalise about ground conditions, or for that matter specific pile design. As such, generic tables, which offset pile type against pile size (and therefore loss of archaeology), can be misleading. For example, some replacement piles may have a lower loading capacity than driven preformed piles of a similar diameter or width, and therefore the area of archaeological deposit damaged would appear to be less for the displacement piles.

Permissible ppv (mm/s)

I Ruins and damaged buildings, protected as monuments 2

II Buildings with visible defects, cracks in masonry 4

III Undamaged buildings in technically good condition 8

IV Well-stiffened buildings (i.e. industrial) 10–40

(ppv = peak particle velocity)

The installation of those driven piles, however, is likely to have a greater impact on the archaeology, with a zone of disturbance at least one pile-width either side of the pile centreline. Furthermore, in cases where it is theoretically possible to use a large single bored pile, multiple driven piles (connected by a pile cap) would usually be needed to provide the same

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load-bearing capacity. Such close grouping of piles makes it more difficult to interpret the intervening deposits, making the effective impact proportionately larger (see below, Pile Groups)

Unless a pile is socketed into a strong load-bearing solid substrate, its bearing capacity will rely principally on shaft friction.The magnitude of shaft friction is a function of soil shear strength and shaft surface area. As soil shear strength usually increases with depth, further enhancing pile capacity, it may be possible to construct a long, slender pile, thereby resulting in less damage to archaeological resources close to the ground surface (A Hyde pers comm; Davis et al 2004, 59).

Pile groups The pile impacts identified above are principally concerned with damage caused by individual piles. Single piles are rarely installed, however. Instead, they are grouped and joined by pile caps, which tie into other building elements (Fig 37). In most cases, isolated piles are likely to be less damaging to the site than grouped piles. This is because the area of sediment enclosed within a pile group, for example three or four piles with a triangular or square arrangement, will be more disturbed. It will be more difficult to interpret the site should it be re-excavated, because it can be hard to access small areas of archaeological deposits within a cluster.

These problems are likely to be exacerbated by the use of driven piles where deposits are modified through down-dragging of sediments. Additionally, any potential hydrological and geochemical impacts are likely to be greater in areas where piles are more closely spaced. Alternatively, pile groups could be located in parts of the site that are not archaeologically sensitive, thereby reducing the number of piles impacting on significant deposits.

Fig 37 (top) Pile group installed by chance adjacent to archaeological deposits. Had the pile group been placed slightly closer to these hypocaust pilae, they would have been severely damaged, and it would have been very difficult to interpret them (photograph courtesy of University of Leicester Archaeological Services (ULAS)).

Pile caps and ground beams Piled foundations do not generally exist in isolation and the presence of pile caps, ground beams and other structural elements needs to be taken into account. Pile caps are generally concrete slabs at the top of the pile, larger than the pile itself and often spanning several piles grouped together. Ground beams are used to connect two or more piles.Their area and depth depends on the distance between piles and where large distances are spanned, the ground beam can be deep and have a significant impact on archaeological deposits. The depth of the existing building slab, and the depth and level of the new basement slab needs to be considered in assessing the impact of ground beams and foundation design. In combination with other foundations, ground beams can be used to span or cantilever over archaeological features allowing piles to be located away from archaeologically sensitive areas. Depending on the use of the building space (including basement requirements), it may be possible to form ground beams within and above the thickness of the ground floor slab, so reducing the below ground impact.

Pile testing To verify the bearing capacity of a pile, a pile test is commonly undertaken prior to the main pile installation phase.The most common form of test is the ‘static’ pile test.This may involve applying a known load to the head of the pile and monitoring its settlement, or advancing the pile into the ground at a known rate and measuring the resisting load. In either case a hydraulic jack is required to apply load to the top of the pile. In turn this needs to jack against some form of rigid structure to provide reaction for the test (Fig 38).Two types of reaction are used, the simpler involving large heavy masses such as concrete or lead weights, which are placed above the test pile.The mass used is often referred to as kentledge. The other method of providing reaction is by means of installing two to four additional piles (reaction piles) around the test pile. Steel beams are then attached to the reaction piles such that they run over the test pile and provide reaction for jacking. Possible impacts on archaeological remains from using kentledge as reaction result from the high near-surface ground loads, which may pose a threat to shallow buried remains. Reaction piles will usually result in additional disturbance unless they form part of the foundation design (see below).

Alternative methods of pile testing are non intrusive and pose no more direct threat to buried archaeological deposits than the pile installation equipment (Fig 39).The most common forms are dynamic and ‘Statnamic’ pile tests (Fig 40). Dynamic pile tests are best suited to driven piles and may be undertaken during the installation phase with no additional plant requirements. Statnamic pile testing does require the mobilisation of specialist plant, but has the benefit of having a mass of only 5% of

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Fig 38 (middle) Static load test (photograph courtesy of Mike Brown).

Fig 39 (bottom) Monitoring equipment being fitted to a driven pile in advance of dynamic pile testing (photograph courtesy of Mike Brown).

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Fig 40 (below) Statnmic pile testing equipment (photograph courtesy of Mike Brown).

the equivalent kentledge and a limited surface footprint. Both dynamic and Statnamic pile testing should be assessed for vibration impact on adjacent structures similar to that required for driven piling.When positioning a test pile, its location in relation to the final construction piles should be considered.Where possible, test piles and reaction piles should be designed to form part of the final construction (working piles) reducing the need for additional piles. On very sensitive sites, this may affect the type of pile test chosen.

Pile testing is covered in detail by the Handbook on Pile Load Testing, produced by the Federation of Piling Specialists (2006).

Contaminated sites and piling Many of the piling issues that concern archaeologists are similar to those that concern the EnvironmentAgency regarding the effects of piling on groundwater. Pile installation on contaminated sites that overlie aquifers can give rise to increased leaching of pollutants to groundwater through vertical pathways created by the piling (Environment Agency 2001;Westcott et al 2003). On sites overlying fractured or fissured rock, or where there has previously been mineral working (ie deep mining), injection of grout (which might impact on shallow archaeological deposits) can also impact further down. At these sites, injection of grout could result in the migration of grout away from the bore over a very large area.Where possible, the archaeological and geotechnical investigations should be carried out alongside each other, to minimise the burden on developers with respect to site characterisation, risk assessment and risk management design.

Mitigating the impacts of piling –a continuous process

The archaeological potential of a proposed development site is usually assessed with a desk-based study and impact assessment, followed by field evaluation.This work can be in response to a development proposal where the impact of the scheme is already known, or to inform a new design. In either case the likely impacts of piling and foundation design should be considered at the earliest stage to allow relevant data to be collected, including foundation design of the existing and previous

buildings on the site.Where foundation schemes are designed without adequate consideration of potential archaeological impacts, unnecessary damage and time delays to the construction programme may result.

During the design process, all archaeological and engineering information should be shared to enable the engineers and architects to design the development and take account of the character and significance of the archaeological remains and any implications involved.This will help to ensure that the most appropriate engineering and mitigation solutions are identified. It is therefore paramount that the character and significance of the archaeological deposits are drawn to the attention of the engineering team at an early stage so that the associated constraints can be considered as part of the design. As piling can impact significantly on archaeological deposits over a wide area, it may also be appropriate to consider the effects of the proposed works on deposits adjacent to the site.

Methods to reduce or avoid disturbance to archaeological remains The most effective method for mitigating the impacts of piling on significant archaeological remains is to adopt an avoidance strategy, whereby piles are located away from archaeologically sensitive areas (Fig 41). In these cases foundations can be designed so that they impact only on the less sensitive areas or on areas of existing disturbance.Where significant remains are present the possibility of providing uninterrupted spaces in the most sensitive areas should be explored.Where this is not possible or feasible then a redesign of the foundations to include raft, ground beam, frame

Fig 41 (bottom) Whatever the piling technique, piles are always likely to damage unknown below-ground archaeological materials. Even in this instance when the piles avoided all of the principle walls, damage has still occurred (© MoLAS).

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supports, or cantilevered structures should be considered.

Reducing the number of piles within groups by increasing the dimensions of the piles should be considered.Where the engineers have been closely involved with the mitigation process throughout, they will be able to design a piling layout that causes the least damage to archaeological remains and, where feasible, avoids the use of pile clusters. Even when a final pile layout is presented on schemes where a process of continuous mitigation has not been followed, it is often possible to agree alternatives with the engineers.

Pile re-use Where a site has an existing piled foundation, re-use should be considered and a feasibility study carried out.This should be done before

demolition or enabling works, because these may damage the foundation.The benefits of pile re-use are obvious since they reduce the need for new foundations. Increasingly, this is a technique that is being used in urban areas where, as the number of times a site is redeveloped increases, so does the number of service trenches, old foundations and other below-ground obstacles (Fig 42).

Over time, the area available for new foundations is dramatically reduced, and in some areas, for example London (with many tunnels), pile re-use may soon be the only feasible option.This problem is exacerbated by the fact that new buildings have a relatively short design-life (Butcher et al 2006a).

In some cases additional piles or foundations will be needed, or the existing piles may need to be strengthened, but even partial pile re-use will result in a reduction in the below-ground impact (Williams 2006). It is also possible to remove piles and re-use the locations for new piles if increased bearing capacity is needed (Hughes et al 2004, 101).This concentrates

damage in areas that have already been affected by piling, although the process of removal is likely to be damaging and methodologies must be considered carefully.

There are a large number of factors that need to be considered in any re-use strategy, including soil conditions, the structural capacity of the existing and new buildings, the character of the archaeological deposits across the site, and the whether pile or pile location re-use is proposed. Further issues include insurance and liability for old foundations, locating technical information about existing piles, testing the capacity of the old piles and the fact that the existing piles may be in the ‘wrong’ place for the new building. Many of these issues have been evaluated by the EC funded project RuFUS, and a handbook for foundation re-use (Butcher et al 2006a), and the proceedings of an international conference on the subject (Butcher et al 2006b) have recently been published. Further information about the RuFUS project is given in the contacts section.

One of the perceived drawbacks of foundation re-use is that each time a site is re-developed, economic pressures dictate that the new building will be larger than that being replaced, which usually means larger foundations.The possibility of over-engineering new piles for future re-use may develop, but this has cost implications, which in the short term may be difficult to justify. However, by investing in piles with greater capacity in the present, substantial cost savings can then be passed on when the site is re-developed in the future. Additionally, it is possible that increased structural loads from larger buildings can be offset by using lighter building materials than were used in the original building.

It is worth stressing that new piles are significantly more likely to be re-used in the future if engineers have full information on the design of these piles. Recommendations for the type of information needed for future re-use are provided in the RuFUS handbook, summarised in the table below.

Fig 42 Ground congestion issues in urban centres severely restrict possible locations for new piles, making foundation re-use a very necessary technique (image courtesy of the RuFUS Consortium 2006, and reproduced from Butcher et al 2006b.

Table 2 Information relating to new piles that should be stored to ensure that pile re-use can take place the next time the building is developed, from the RuFUS handbook (Butcher et al 2006a)

Program stage Design stage Construction stage Building operation

Geological information Geotechnical information Groundwater level Groundwater quality Contaminated soil Site conditions

Design philosophy Design codes Design calculations Necessary bearing capacity Force combinations applied on each pile Pile data Settlement limitations Protocol for foundation records

As-built documents Non conformance reports Construction documents Programme of piling works Plant and equipment Test piling Working documents Site records

As-built drawings Maintenance records Environmental changes Inspections Pile behaviour Service life measurements Structural alterations

Pile installation records Effects on nearby foundations

and structures Results from monitoring

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Understanding the zone of disturbance Avoidance strategies should be considered on a site-by-site basis, taking into account the scale and nature of the development and the archaeological potential. All piling operations will result in the physical destruction of archaeology directly in the path of the pile and, while it is accepted that destruction will occur, there has been much discussion of what constitutes an acceptable level.

A study into development and archaeology in the City of York (Ove Arup and Partners and York University in association with Bernard Thorpe 1991) proposed that the use of single bored (replacement) piles centred on a 6m by 6m grid would result in the destruction of between 2% and 5% of the area within the footprint of the construction while still maintaining the ‘legibility’ of the deposits.The value of 5% has, inadvertently, gained credence as the maximum permissible area of destruction.This does not take into account the cumulative effect of successive developments. Developments on archaeologically sensitive sites should strive to achieve lower values.

The York study (ibid 1991) also highlighted the possibility that disturbance to a zone larger than the size of the pile might occur. Recorded observable impacts from square displacement piles are extremely variable, ranging from no perceptible change through to distinct zones of impact where the integrity of the stratigraphy equal to at least twice the width of the pile has been compromised.This is clearly an area where further study is required.When calculating the likely level of impact from displacement piles, it is suggested that an area of impact equal to twice the width of the pile (ie one pile width either side of the pile centreline) is assumed which equates to a four­fold increase in the area of pile impact and it is this value that must be factored in when calculating ‘loss’ of archaeology.

Local authority planning and archaeological officers need to be mindful of the cumulative impact of re-development on a site which makes later interpretation of a site more difficult. In these cases, foundation re-use, both of the existing foundations or their locations would be a beneficial mitigation method. In other cases, there should be recognition that excavation would be better than any further attempts at preservation in situ.

Pre-augering and probing for obstructions One method to reduce potential physical damage to sediments from preformed displacement piles is to pre-auger the pile locations.This technique has recently been trialled on two sites which have subsequently been excavated archaeologically (Davies 2003; Rayner 2005). In both cases the excavation demonstrated that the impact of the subsequent displacement piling was limited to the area already disturbed by the pre-augering.

In order that this technique is successful, it is recommended that the auger diameter is equal to the diagonal of the pile and augered to below the depth of known archaeology (Fig 43).The material disturbed in pre-augering should not be removed from the hole, but left in situ while the auger is rotated out in the opposite direction.

When replacement (bored) piles are to be installed on sites where substantial structural remains are suspected (stone walls/foundations, etc), then the piling contractor should be made aware of this issue, and a tool capable of cutting through such obstructions should be used. If CFA or preformed driven piling is undertaken, and obstructions are encountered, two options are available: either the removal of the obstructions through excavation from the surface or the relocation of the pile(s). Removing obstructions in advance of CFA or driven piling may involve probing or pre­augering with diamond or chisel cutting tips; however, contractors may more usually engage in large-scale machining. Both options result in a collateral loss of archaeological integrity as the area around an obstruction is checked thoroughly for obstructions.

Two archaeological watching briefs carried out during piling at a monastic site known to contain walls demonstrated that locating places suitable for piles on complex urban sites is fraught with difficulties. Although the piling design for the site had sought to reduce the impact on archaeological remains to well below 5%, unauthorised probing took place, which was monitored archaeologically. As a result the final disturbance of the uppermost (medieval) deposits was much greater, 17% in one of the two cases, which resulted from the need to extend the archaeologically recorded areas in order to try to understand the remains. However, the information gained from these

small trenches placed across the site was limited, and the plan of the friary remains was still difficult to interpret and therefore did not enable research questions to be fully addressed (McDaid 2006a; 2006b). In this case, more extensive area evaluation in advance of the design of the pile layout might have provided a piling plan which avoided major obstacles and allowed for more interpretation of the archaeology (M McDaid pers comm), although this would have had financial and time implications for the developer (M Jones pers comm).

Locating obstructions is potentially a very damaging stage of construction as, quite reasonably, developers seek to avoid unexpected ground conditions.This may also not be part of the main piling programme, but included within demolition or enabling contracts. A methodology detailing steps to be taken when encountering obstructions should be prepared for each site and in some cases it may be appropriate for an archaeologist to be present to ensure it is adhered to.

Fig 43 Recommended diameter for pre-augering (circle) shown along with the square pile, with the same distance across the diagonal as the diameter of the auger hole.

Piling and waterlogged deposits Understanding the full impacts of piling on waterlogged deposits is complex, and requires a thorough knowledge of the site hydrology. The chemical impact of pile concrete from replacement piles on waterlogged deposits is not yet fully understood, and although two recent field tests did not identify impact beyond 0.5m from the pile, there still remains the potential that some chemical damage may occur. During the time that the pile cures, there is a potential risk that the migration of chemicals from the pile grout/cement will affect the local sub-surface groundwater.The impact of this will to a large degree depend upon the nature of the waterlogged deposits, and the rate of groundwater flow. Deposits with a high hydraulic conductivity, such as gravel, may have fairly rapid groundwater movement, but organic-rich, peat-like deposits typically have a low hydraulic conductivity, meaning groundwater movement will be limited and therefore the potential risk of contaminant transport is significantly reduced.This risk could be further reduced by the installation of permanent casing.

Concerns exist regarding the possibility of piles puncturing impermeable layers that contribute to the preservation of waterlogged deposits, particularly in urban environments, such as York, where there are perched water tables. Mitigation for development where water­logging is known to occur above the natural groundwater level should include an appraisal of the proposed foundation design and a consideration of whether an avoidance strategy can be adopted. Recent research (Hird et al 2006) indicates that the most important factor is the thickness of the aquitard (the impermeable layer restricting groundwater flow).Where piling is the only option on

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waterlogged sites with perched water tables, then the use of permanent rather than temporary casings on replacement piles should be considered, as the removal of these casings may also disrupt the aquitard.

Cofferdams constructed from augered secant or driven sheet piles, whether used to control water ingress during construction or in flood defence barriers, may impact indirectly upon waterlogged archaeological remains by altering water levels. Mitigation must include a consideration of groundwater flow and possible effects on the archaeology.Where the barriers are long-term, there is every possibility that waterlogged deposits may effectively be cut off from hydraulic recharge and therefore ‘preservation by record’ (excavation) would be the only option.

Piling and burial grounds Burial grounds are a good example of where an avoidance strategy should be implemented or full excavation undertaken (Fig 44). It should be stressed that the planned use of piled foundations on a burial ground will not normally be permitted by the Ministry of Justice. However, while avoidance of disturbance is the preferred option, the archaeological excavation of all burials within the footprint of the development will often be necessary. Raft foundations and ground beams may be feasible (see for example case provided by Shilston and Fletcher 1998), although it should be emphasised that further research is needed to evaluate the impact of raft foundations, and compression in general. For further information on the implications of piling for burial ground archaeology please refer to the Guidance for Best Practice for Treatment of Human Remains Excavated from Christian Burial Grounds in England (The Church of England /English Heritage 2005). Similar considerations should be given to any non-Christian burial grounds.

Fig 44 Piling in cemeteries is likely to cause significant damage, as can be seen on the cover image. In this case, the pile cap has only just avoided this skeleton (image courtesy of the University of Leicester Archaeological Services (ULAS)).

Reporting Our understanding of piling impacts and the best mitigation methods comes from experience.The more sites where previous construction impacts can be assessed, the more our experience will grow, but this will only happen with comprehensive dissemination of information. It is therefore recommended that brief summaries of the methodology and rationale are produced for sites where preservation in situ schemes through piling have been developed. Planning and archaeological officers may wish to make this part of a condition relating to the in situ preservation strategy.

To assist in the dissemination of knowledge, English Heritage has created a mailbox so anyone involved with piling and archaeology can submit case studies, images and methods which can then be integrated into future publications (where appropriate).The email address is: piling@english-heritage.org.uk

It is also imperative to the success of foundation re-use schemes that all available design and construction data for current foundations are stored in a suitable location such as the local Historic Environment Record. Data would include the final pile locations, loading capacity, test results, and as-built drawings. Other information such as the engineers design report, contractors method statements and more detailed designs should form part of the site archive.

Summary A summary of mitigation measures is presented in Table 3.

Table 3 Summary of Pile Mitigation

Pile Type Mitigation

All pile types Adopt ‘avoidance strategy’ and avoid use of piles in areas of archaeological sensitivity where possible. Burial grounds should not be piled. Where piling is unavoidable, limit extent of physical destruction as far as possible (consider all ground interventions including ground beams

Large displacement piles Zone of impact is potentially greater than diameter of pile, therefore calculate percentage loss of area in building footprint using four times the pile area, unless there is evidence of the impact of past piling activity recovered through excavation.

Small displacement piles Sheet - If waterlogged remains are present, assess potential impacts on groundwater flow and recharge of deposits. Consider long-term monitoring of water-table and water chemistry. H-section - Not recommended for waterlogged deposits due to possible migration and oxygen ingress.

Replacement piles Consider use of suitable cutting tools where obstructions are likely to be encountered. For secant walls see above for sheet piles CFA - Avoid on sites where structural remains likely

Vibro techniques Require further investigation, but are likely to be extremely damaging to archaeology and should be avoided where possible.

Case studies

Pile pre-augering: JunXion, Lincoln (see Figs 27 and 28) The City Archaeologist originally requested that only CFA piles with permanent liners should be used, due to concern about down-dragging of deposits by driven piles.The requirement for the permanent liner was due to the high groundwater level on site. Excavations at an adjacent site (Steane et al 2001) had revealed deeply buried and well-preserved timber associated with revetting and land reclamation in the form of wattle hurdles and dumped material dating from the Roman period onwards. Deep evaluation was not undertaken as no part of the building other than the piles was likely to impact on this material, and parts of the site were also contaminated with oil and diesel fuel (M Jones pers comm).

Despite the foundation recommendations by the City Archaeologist, the developer was keen to use preformed displacement piles, partly because they would be much cheaper.The impacts of these piles on archaeological deposits was outlined, and it was agreed that it would be inappropriate to consider a loss through piling based only on the actual width of the piles. A figure of twice the width of the piles was used, because it was felt that this would take into account the potential

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deformation of adjacent deposits. Calculations made using this larger figure indicated that the area of deposit loss was still below 5%, which it had already been agreed would be acceptable (given the character and importance of the archaeological remains on this site).

The piling contractor believed that the actual damage from the use of driven piles would be lower if the locations were pre-augered to below the depth of the archaeology. They particularly wanted to use this technique along one side of the site to reduce piling vibration on the printing presses of the local newspaper housed in the adjacent building. The methodology entailed pre-augering the pile location with a 350mm diameter auger to a depth of 3–4m then withdrawn whilst rotating in the opposite direction.This left the soil in the ground, but disrupted it sufficiently to make the insertion of 250mm square piles easier.The auger size was chosen to match closely the distance across the pile diagonal (353mm). As the technique had not apparently been used before on an archaeological site,

it was agreed that it could be used along one side of the site, providing an evaluation of its impact on archaeology was carried out.

Following excavation, no evidence of disturbance outside the area of the auger (ie 350mm) could be identified. Since the potential damage estimated for the driven piles on this site was twice the width of the pile (c 500mm), this therefore represented a reduction in the potential damage that might have occurred from driven piling alone. However, evidence from the pile driven without first pre-augering indicated that down-dragging of material and its impact on the deposits was also limited to a zone no greater that 350mm. On this basis, for this particular site, it was decided that there was no need to pre-auger the other piles on the site (except those adjacent to the printing press).

Steel screw piles: Salisbury This technique, recently adapted from 19th­century marine engineering, was used on a

sensitive archaeological site in Salisbury (Fig 45).The piles were made up of a number of curved spirals of steel of varying diameters connected to a central shaft.These piles, which had a 250kN capacity (T Sheward pers comm), were screwed into the ground to depths of 5m (Sheward 2003). Benefits were that it was unnecessary to remove spoil associated with any piling operation, or to bring piling materials to the site through the city’s narrow streets. Additionally, the piles can be removed by unscrewing at a later date theoretically causing very limited damage to below-ground deposits.

Fig 45 (left) Screw piles being installed (photograph courtesy of Tim Sheward, Roger Bullivant).

Fig 46 (below) Ground plan showing location of previous and new piles (courtesy of York Archaeological Trust).

Existing Building

Existing pile foundation

Gas sampling borehole 0 10 metres North

Former Victoria House, Micklegate, York

Figure 2 Trench and borehole location

Existing Building

9 11 4 1 8 7

10 65 17

3 2 16

12 13 wall 4006

14 15

core wall 4005 4006

wall 4005

Alignment of new/New pile foundation existing ground beams

Areas of archaeologicalobservations with designated area number

section 1

section 3 section 2

wall 6008

wall 6001

wall 7039 wall 7044

wall 7007

Trench 7

Trench 6 Trench 8

Trench 4

Trench 5

Trench 1 Trench 2 Trench 3

Fig 47 (above) Exterior of the Ramada Encore Hotel (photograph by Andy Hammon).

Pile re-use: Ramada Encore Hotel, Mickelgate, York The previous building on the site was the offices of the Yorkshire Co-operative Society, constructed in the 1960s.The site was acquired by a developer to build a hotel upon. During discussions with the City Archaeologist, the developer was informed of the likely archaeological potential of the site, which was situated within the medieval town walls, not far from the riverside, and therefore likely to

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contain well preserved organic material. On the basis of that discussion, the developer produced a plan to re-use the foundations of the existing building, thereby reducing the potential need for, and cost of archaeological evaluation (Figs 46 and 47).The scheme, which included the re-use of all 110 previous piles, needed a further 17 installed in three discrete locations.This meant that over most of the rest of the site, no ground disturbance occurred. Any below ground impact was further mitigated because the building was constructed on the existing ground slab with archaeological recording during the installation of services and pile caps, none of which were deep enough to encounter significant archaeological deposits.

This scheme was very successful, mainly because the potential for re-use had been highlighted early enough in the design phase of the scheme, and was led by the developer, who was keen to reduce the risk to the scheme of having to deal with archaeological material (Williams and Butcher 2006).

Pile avoidance and redesign: 43 The Highway, Shadwell, London An exceptionally well-preserved Roman building was discovered during excavation in advance of development (Fig 48). Roman remains had been anticipated following evaluation, but not the quality of the building and extent of its survival. It was considered by the archaeological curator to be a find of national significance and therefore preservation in situ was recommended and agreed by all parties. However, planning permission subject to a condition to archaeologically record and excavate the site had been granted for a multi-storey residential block of apartments.

The site is located in the Thames floodplain, on inherently unstable alluvial sediments, requiring

substantial piled foundations through river silts and gravels. Discussions took place on exactly which aspects of the Roman archaeology needed to be preserved in situ and what, if anything, could be preserved by record. It was decided that all intact structural elements needed to be preserved while some spaces between walls could be fully excavated, recorded, backfilled and then piled through.The use of detailed digital plans of the archaeology was extremely important to compare with the proposed foundation plan including pile locations.The foundations were redesigned to allow development while retaining the building intact.The proposed CFA technique was retained and no pile dimensions had to be changed. Piles were relocated to areas between the Roman walls and hypocaust pilae, with as much clearance as possible between pile locations and the Roman building.The building was backfilled to an agreed specification involving geotextile, inert sand, and then graded spoil from the site. CFA piles were then carefully located and installed, securing the safety of the building.

Fig 48 The Shadwell bathhouse (© Pre-Construct Archaeology Ltd).

Research strategy

Since the introduction of PPG16, there has been a need for clear guidance on how to achieve preservation for in situ schemes. During that time three conferences on ‘Preserving Archaeological Remains in situ’ (PARIS) have taken place (Corfield et al 1998; Nixon 2004). More research is, however, needed to address issues including:

• impact of piles in a range of soil and archaeological deposits

• impact of piles on hard ‘floor’ levels buried beneath the ground

• impact of vibration and compressive forces in stratified deposits

• possibilities for the re-use of old piles (or re-drilling old pile locations)

• impacts on deposit hydrology, specifically: • impact of piling on impervious clay

layers and water flow • impact of concrete/grout upon

groundwater chemistry and artefact/ecofact stability

• impact on hydrology (of archaeological deposits) of adjacent sites

• impact of piling upon soil microbiology • stability of metal sheet piles within aerobic

deposits • impact of other foundation types

This research will need to be both laboratory-and field-based.

There is also a need to evaluate areas on sites where past construction activity has impacted upon archaeological deposits and there have been adverse effects.This will enable an understanding of actual site-based impacts to be collated, and is a recommendation included in the City of London Corporation Planning Advice Note 3, Archaeology Guidance (Corporation of London 2004).

Targeted field evaluations should be carried out on:

• CFA piles • bored piles, particularly investigating the

impact of installation and removal of temporary casing

• auger displacement piles • removal of existing piles • vibro techniques

Parameters that should be recorded are:

• depth and lateral extent of physical effects • accurate sediment description, preferably

including particle size analysis • moisture and organic content • number and size of inclusions • assessment of compression • distance of grout migration from cast in situ

piles • any microbiological, chemical or

groundwater impacts from grout migration

The collection of this type of information will be needed to inform future site-based and generic mitigation strategies.

Best practice: summary ofguidance

There is no one method of piling that should be adopted for all archaeological sites. As this guidance note has demonstrated, there are engineering and archaeological reasons why a particular piling technique would be employed on any given site.The following recommendations form a best practice guide.

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Early involvement

• Pile design should be considered early in the development programme.

• Feasibility studies for foundation re-use should be carried out.

• Piling will always be destructive, and less damaging foundation techniques should be considered.

• The implications of foundation damage on archaeology should be considered at all stages of evaluation, not just in the mitigation strategy after all other evaluation phases have been completed.

• Where preservation in situ is to be achieved by piling, site evaluation needs to be sufficiently detailed so that the impact of piling on all archaeology across the site is fully understood.

• Site characterisation should include a detailed model showing the depth of archaeological deposits, and discussion of any areas that might contain obstructions to piling.

• To ensure that the recommendations listed above can be achieved, close working between the engineers and archaeologists will be essential from the outset.

Pile impact

• New piling impact on the site’s archaeology should be kept to a minimum, and a loss of no more than 2% of the site should be the target.When all other engineering works are also taken into account, such as services and lift pits, a maximum of 5% of the total site should be seen as the upper limit of loss from foundation construction.

• Consideration must be given to the cumulative impact of foundations.There will come a point when the piling impacts from previous construction have already compromised the future readability of the deposits.

• Where sites have been adequately characterised, it should be possible to avoid the most archaeologically sensitive areas of the site through careful pile placement and appropriate load-bearing spanning structures.

• Pile clusters of more than two piles should be avoided wherever possible, as archaeological deposits within pile groups of three or four piles will be un-interpretable in the future.This can be achieved by increasing the diameter or depth of the piles. It may be appropriate to excavate and record the whole area of the pile cap.

Pile choice

• Pile choice will ultimately depend on the engineering requirements of the building, but it should also be influenced by the archaeological mitigation strategy.

• The key factor is that the zone of destruction for each pile type is recognised by the design team, and this will vary depending on ground conditions and type

of archaeological deposit. Consideration should be given to the physical impact, and also any impact on the site hydrology, chemistry and microbiology.

Displacement piles • It is recommended that where square

driven piles are used, that they are assumed to have an area of impact twice the width of the cross-section (and so four times the area).

• Thus, a 0.25m wide driven pile will not just damage 0.0625m

2 (the area of the pile),

but 0.25m2 (four times the area of the pile).

• For a 25m by 25m building using piles 0.25m

2, then in order to achieve no more

than a 2% loss of archaeology, 50 piles could be used as follows:

area of each pile = 0.0625m2

the potential impact of each pile (twice the area) = 0.25m

2

The cumulative pile damage = single pile area (0.25m

2) x number of piles (50) = 12.5m

2,

which is 2% of the total area of the site.

• It should be noted that in certain ground conditions the zone of effect of a driven pile can be smaller than that suggested above.The onus should rest with the developer to demonstrate this. Evaluation of previous foundations where they exist on a site will help to establish specific conditions.

• To achieve more certainty about the total area of damage from driven piles, and to reduce the amount of damage, pile locations can be pre-augered before the pile is driven.

• An auger with a diameter the same size as the diagonal of the square pile should be used, and soil should be left in the pile hole, and not removed.

• The impact of full displacement augered piles has not been evaluated, but it is likely that damage to surrounding deposits would be at least as much as is seen with driven preformed piles.

Replacement piles • The area of physical loss is designed to

relate purely to the area of the pile, but exceptions occur when the borehole sides collapse (not likely for CFA piles), or when concrete from the pile migrates into the unconsolidated deposits adjacent to the bored/augered hole. In both cases these impacts can be mitigated by installing temporary or permanent casing (Fig 49).

• In all cases a thorough archaeological evaluation and characterisation of the site should be undertaken to indicate the likelihood of encountering buried structures (either wooden or stone/brick).

• Where these cannot be avoided by careful placement of the piles, a tool capable of cutting cleanly through such obstructions should be used.

Fig 49 Concrete migration into a void, in this case from the pile cap (image courtesy of the University of Leicester Archaeological Services (ULAS)).

Vibro techniques • The impacts of vibro techniques are largely

unknown.The onus should lie with the developer to demonstrate clearly the amount of archaeological material that would be lost. If risk is perceived to be significant, then these techniques should not be considered within a programme of preservation in situ.

Pile installation

• To avoid damage during piling, it is recommended that a detailed methodology for the piling works and enabling works is drawn up and agreed by all parties.

• To ensure that this plan is adhered to, it may be appropriate to maintain an archaeological presence on site during the piling works.

Records for the future

• To aid future decisions, it is essential that a record of the foundations, as built, is kept with the rest of the site archive.

• This should include a final pile plan, and loading details, as well as records from any pile tests.

• These should also be logged with the local Historic Environment Record.

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Contact details

Within English Heritage the first point of contact for general archaeological science enquiries should be the regional science advisor, who can provide independent non-commercial advice.

East of England (Bedfordshire, Cambridgeshire, Essex, Hertfordshire, Norfolk, Suffolk) Dr Jen Heathcote English Heritage regional office 24 Brooklands Avenue Cambridge CB2 2BU tel 01223 582759 mobile 07979 206699 e-mail jen.heathcote@english-heritage.org.uk

East Midlands (Derbyshire, Leicestershire, Rutland, Lincolnshire, Nottinghamshire, Northamptonshire) Dr Jim Williams English Heritage regional office 44 Derngate Northampton NN1 1UH tel 01604 735 400 mobile 07801 213300 e-mail jim.williams@english-heritage.org.uk

London Dr Jane Sidell English Heritage regional office 1 Waterhouse Square 138–142 Holborn, London EC1N 2ST tel 0207 973 3000 mobile 07811 513025 e-mail jane.sidell@english-heritage.org.uk

North East (Northumberland, Durham [including former Cleveland],Tyne &Wear, all of Hadrian’sWall) Mrs Jacqui Huntley Department of Archaeology University of Durham Science Laboratories Durham DH1 3LE tel and fax 0191 374 3643 mobile 07713 400387 e-mail Jacqui.huntley@english-heritage.org.uk

English Heritage regional office Bessie Surtees House 41–44 Sandhill Newcastle upon Tyne NE1 3JF tel 0191 269 1586

North West (Cheshire, former Greater Manchester, former Merseyside, Lancashire, Cumbria [excluding Hadrian’s wall: see North East]) Dr Sue Stallibrass University of Liverpool School of Archaeology, Classics and Oriental Studies (SACOS) William Hartley Building Brownlow Street Liverpool L69 3GS tel 0151 794 5046 e-mail Sue.Stallibrass@liv.ac.uk

English Heritage regional office Canada house 3 Chepstow Street Manchester M1 5FW tel 0161 242 1400

South East (Kent, Surrey, Sussex, Berkshire, Buckinghamshire, Oxfordshire, Hampshire, Isle of Wight) Dr Dominique de Moulins Institute of Archaeology 31–34 Gordon Square London WC1H 0PY tel 0207 679 1539 e-mail d.moulins@ucl.ac.uk

English Heritage regional office Eastgate Court 195–205 High Street Guildford GU1 3EH tel 01483 252000

South West (Cornwall, Isles of Scilly, Devon, Somerset, Dorset,Wiltshire, Gloucestershire, Bath and NE Somerset, Bristol, South Gloucestershire, North Somerset) Ms Vanessa Straker English Heritage regional office 29 Queen Square Bristol BS1 4ND tel 0117 975 0700 mobile 07789 745054 e-mail Vanessa.Straker@english-heritage.org.uk

West Midlands (Herefordshire, Worcestershire, Shropshire, Staffordshire, the former county of West Midlands,Warwickshire) Ms Lisa Moffett English Heritage regional office 112 Colmore Row Birmingham B3 3AG tel 0121 625 6820 mobile 07769 960022 e-mail lisa.moffett@english-heritage.org.uk

Yorkshire Region (North Yorkshire, former South and West Yorkshire and Humberside [East Riding of Yorkshire, Kingston-upon-Hull, North Lincolnshire, and North East Lincolnshire]) Dr Andy Hammon English Heritage 37 Tanner Row York YO1 6WP tel 01904 601 983 e-mail andy.hammon@english-heritage.org.uk

RuFUS The RuFUS (Re-use of Foundations for Urban Sites) project has now finished, but details of the project and the publications produced from it can be found on the project website: www.reuseoffoundations.com

Glossary

anoxic used to refer to a deposit in which oxygen is virtually absent

aquitard an impermeable layer restricting groundwater flow between aquifers

arisings spoil generated and brought up through groundworks/drilling

bentonite an absorbent aluminium silicate clay mineral used in slurry form as a drilling mud. It has a has a specific gravity of about 1.2 thus is sufficient to stop water and soil ingress.

casing generally a tube used to line the pile hole; usually of metal and removed following piling

cathodic protection an electrochemical process used to protect metals from corrosion in water/aquatic environments

cohesive/cohesionless soils terms used to refer to firm or loose soils, i.e clay rich (cohesive) or gravel (cohesionless)

deformation generally used to refer to a change in shape, in this case, usually to a soil or sediment, resulting from applied force

displacement generally lateral movement of soil during insertion of a pile

drilling fluids fluids used to aid the drilling process, often a form of slurry, bentonite or even water

end bearing a piling system where most of the load is carried by the base (end) of the pile

exothermic a chemical reaction which produces heat

helical a helical pile is corkscrew shaped; a central bar with a series of pitched plates attached

high slump concrete has a high water to cement ratio, making it a highly workable material.

hydraulic conductivity is a measure of the way and speed water passes through soils/other mediums

Hz Hertz

kentledge a form of incremental pile loading used for testing piling

kN = 1000 Newtons A Newton is the force required to accelerate 1kg mass at 1m/s2. An apple exerts a force of approximately one Newton, and a mass of one tonne equates to 10kN in the Earth's gravity field.

particle velocity the velocity at which the ground vibrates. It is measured in millimeters per second. Peak particle velocity has been accepted as an important indicator of structural damage.

perched (water table) water held above the real water table, usually through the presence of an impermeable layer

secant technically a line passing through two points of a curve – in this case, a secant wall is a line of intercutting piles

shear strength this is the maximum stress which can be sustained before a material will rupture, or fail in shear

sleeving a covering for the pile, generally permanently left in the ground; can be paper, metal, plastic etc; sometimes used for guidance during drilling

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statnamic a rapid load testing method for piles which may be used as an alternative to static or dynamic tests

tie-back an anchorage or the tie rod connected to it which may be used to support walls and other structures

underream an enlarged pedestal cut out of the soil at the base of a pile.This is usually done with a cutting tool, which can be expanded and rotated at the base of the pile shaft.

unstable soils sands and gravels which are not self-supporting and therefore liable to collapse into a bored hole

References

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Williams, J and Butcher, A P 2006 ‘Foundation re-use as a mechanism for the preservation of buried cultural heritage in urban centres: how new engineering research helps limit archaeological damage’, in Proceedings of the 7th European Conference 'Sauveur' Safeguarded Cultural Heritage, Understanding and Viability for the Enlarged Europe, Prague 31st May–3rd June 2006: available on-line at http://www.arcchip.cz/ec-conference/ programme.php

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Authorship and acknowledgements This guidance note has been written by Jim Williams (EH), Jane Sidell (EH) and Ian Panter (formerly EH, now York Archaeological Trust). It has benefited from comments on the three draft texts from people working within all aspects of engineering and archaeology, to whom the authors are exceptionally grateful: Mike Collins (EH, on behalf of AMIG),Tom Cromwell, Andrew David, Stuart Ellis,Terry Girdler, Jacqui Huntley, Edmund Lee, Simon Mays, Lisa Moffett, Dominique de Moulins, Sebastian Payne, Jennie Stopford, Roger Thomas, Humphrey Welfare (all at English Heritage), Rab Fernie and Peter Borne-Webb (Cementation Foundations Skanska), Michael Brown (University of Dundee),Tony Butcher (BRE), Mike Corfield, Glyn Davies (ARCUS), Mat Davis, James Dinn (Worcester City Council on behalf of ALGAO), Brian Durham (Oxford City Council and on behalf of ALGAO), Allan Hall (University of York / EH), Bob Handley (Aarsleff), Charles Hird and Adrian Hyde (University of Sheffield), Richard Hughes (ARUP), Sarah Hughes (ARUP),Taryn Nixon (MoLAS), Jonathan Smith (Environment Agency), Kathryn Stubbs (City of London Corporation),Tony Suckling (Stent Foundations Ltd), Matthew Warner (Amplus Ltd), Lee White (Durham County Council),Tim Bradley (Pre-Construct Archaeology Ltd).

Thanks must also go to those that have supplied us with illustrations: Mark Allen (Allen Archaeological Associates), Peter Bourne-Webb (Cementation Foundations Skanska), Cheryl Blundy (Pre-Construct Archaeology Ltd),Tim Bradley (Pre-Construct Archaeology Ltd), Mike Brown (University of Dundee), Andy Chopping (MoLAS), Ann Christensson (Norwegian Directorate of Cultural Heritage), Hal Dalwood (Worcestershire Historic Environment and Archaeology Service), Glyn Davies (ARCUS), Keith Emmett (University of Sheffield), Gabriel Guigue (Trace Parts S.A.), Andy Hammon (English Heritage),Tim Higgins (University of Leicester Archaeological Services), Charles Hird (University of Sheffield), Adrian Hyde (University of Sheffield), Peter Moore (Pre-Construct Archaeology Ltd),Taryn Nixon (MoLAS),Tim Sheward (Roger Bullivant), Martyn Singleton (Keller Ground Engineering), Kathryn Stubbs (City of London Corporation), John Tate (University of Leicester Archaeological Services), Gary Taylor (Archaeological Project Services),Yvonne Wilder (IHS BRE Press).

Published July 2007

© English Heritage 2007 Edited and brought to press by David M Jones, English Heritage Publishing Designed by Amy Slater Printed by [PRINTER TO ADD] Printed on recycled paper

Product Code 51352

English Heritage is the Government’s statutory advisor on the historic environment. English Heritage provides expert advice to the Government about all matters relating to the historic environment and its conservation.

For further information and copies of this leaflet, quoting the Product Code, please contact:

English Heritage Customer Services Department PO Box 569 Swindon SN2 2YP

telephone: 0870 333 1181 fax: 01793 414926 e-mail: customer@english-heritage.org.uk

Front Cover: (Significant damage to a cemetery burial caused by piling, image courtesy of University of Leicester Archaeological Services)

Back Cover: (Roman well from the Walbrook Valley, London, showing minimal damage, image courtesy of Pre-Construct Archaeology Ltd)